U.S. patent number 5,554,236 [Application Number 08/456,603] was granted by the patent office on 1996-09-10 for method for making low noise signal transmission cable.
This patent grant is currently assigned to W. L. Gore & Associates, Inc.. Invention is credited to William P. Mortimer, Jr., David T. Singles, Grant Walter.
United States Patent |
5,554,236 |
Singles , et al. |
September 10, 1996 |
Method for making low noise signal transmission cable
Abstract
An improved low noise electrical signal transmission cable is
disclosed. The cable employs an insulative layer of expanded
polytetrafluoroethylene (PTFE) which is bonded in fixed relative
position with a surrounding shield layer through use of an
adhesive, such as fluorinated ethylenepropylene (FEP) or
perfluoroalkoxy polymer (PFA). The bonding process reduces the
separation of layers which can sometimes occur with expanded PTFE
insulative cables and avoids the generation of unwanted
triboelectric signals that can result from such separation.
Inventors: |
Singles; David T. (Newark,
DE), Walter; Grant (Newark, DE), Mortimer, Jr.; William
P. (Conowing, MD) |
Assignee: |
W. L. Gore & Associates,
Inc. (Newark, DE)
|
Family
ID: |
22765845 |
Appl.
No.: |
08/456,603 |
Filed: |
June 1, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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206319 |
Mar 3, 1994 |
5477011 |
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Current U.S.
Class: |
156/52;
156/229 |
Current CPC
Class: |
H01B
11/1066 (20130101); H01B 11/1834 (20130101) |
Current International
Class: |
H01B
11/02 (20060101); H01B 11/18 (20060101); H01B
11/10 (20060101); H01B 013/22 () |
Field of
Search: |
;156/47,51,52,53,56,244.11,244.12,229 ;427/118,117,119 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0061829 |
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Oct 1982 |
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EP |
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93/15512 |
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Aug 1993 |
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WO |
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Other References
"Special Audio, Communication, and Instrumentation Cables Low
Triboelectric Noise Coaxial Cables," Belden Cooper Ind.,
Date-unknown, p. 260. .
"Mini-Noise" Coaxial Cable; Malco Microdot Inc., pp. 16-19,
date-unknown. .
"Extrustion Foaming of Coaxial Cables of Melt-Fabricable
Fluorocarbon Resings," Stuart K. Randa, Du Pont Co., Nov. 18,
1981..
|
Primary Examiner: Ball; Michael W.
Assistant Examiner: Lorin; Francis J.
Attorney, Agent or Firm: Genco, Jr.; Victor M.
Parent Case Text
RELATED APPLICATIONS
The present application is a division of U.S. patent application
Ser. No. 08/206,319 filed Mar. 3, 1994 now U.S. Pat. No. 5,477,011.
Claims
The invention claimed is:
1. A method of producing a low noise electrical signal transmission
cable which comprises:
providing a conductor;
providing a porous insulative layer comprising at least in part
polytetrafluoroethylene;
laminating said insulative layer to a thermoplastic film layer at a
temperature above a melt temperature of the thermoplastic film
layer:
stretching said laminate at a temperature above the melt
temperature of the thermoplastic:
providing a shield layer surrounding the insulative layer; and
bonding the laminate in fixed relative position to the shield to
avoid separation and movement between the laminate and the shield
during use thereby reducing the formation of triboelectric currents
resulting from said movement.
2. The method of claim 1 which further comprises:
bonding the insulated layer to the conductor to resist movement
between the insulative layer and the conductor during use.
3. The method of claim 1 which further comprises:
mounting a semi-conductive layer between the insulative layer and
the shield layer; and
bonding the insulative layer to the semi-conductive layer and
bonding the semi-conductive layer to the shield layer in order to
retain the insulative layer in fixed relative position with the
shield layer during use.
4. The method of claim 3 which further comprises:
bonding the insulative layer to the conductor to resist movement
between the insulative layer and the conductor during use.
5. The method of claim 4 which further comprises:
bonding the conductor, insulative layer, semi-conductive layer, and
conductive layer together with a fluoropolymer adhesive after the
cable has been assembled by applying heat to the assembled
cable.
6. The method of claim 1 which further comprises:
providing an insulative layer comprising at least in part a polymer
selected from the group consisting of expanded
polytetrafluoroethylene (PTFE) or foamed fluorinated
ethylenepropylene (FEP);
laminating a layer of fluorinated ethylenepropylene (FEP) adhesive
to the insulative layer;
embedding perfluoroalkoxy polymer (PFA) adhesive within the polymer
material; and
applying heat to the FEP and PFA adhesives to bond the insulative
layer in fixed relative position to the shield layer.
7. The method of claim 6 which further comprises:
surrounding the insulative layer with a semi-conductive layer;
wherein the FEP and PFA adhesives serve to bond together the
conductive layer, the insulative layer, and the conductor.
8. The method of claim 1:
wherein the insulative layer comprises at least in part a polymer
selected from the group consisting of expanded
polytetrafluoroethylene (PTFE) or foamed fluorinated
ethylenepropylene (FEP).
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates electrical cables for transmitting
electrical signals, and especially cables which transmit with high
signal fidelity.
2. Description of Related Art
Traditional low signal level cable designs (also referred to as
"low noise" and "low triboelectric effect" wires or cables) have
used full density materials such as polyethylene or a fluoropolymer
such as fluorinated ethylenepropylene (FEP) or
polytetrafluoroethylene (PTFE) for their dielectric insulation.
These dielectrics are then further insulated with a layer of carbon
impregnated PTFE or silicon. This additional layer acts as a
semi-conductive layer to attenuate charges originating in shielding
for the cable.
Unfortunately, full density insulators like PTFE suffer from a high
capacitance which can limit their performance in certain high
demand applications, such as in high gain audio amplifiers,
oscilloscope probes, piezoelectric components (e.g., microphones,
accelerometers, and eddy current sensors). Capacitance (C) in this
instance is defined as: ##EQU1## Where .sub.eft =effective
dielectric constant; D=diameter over the dielectric; and d=diameter
over the center conductor. Typical capacitance values in
commercially available products today range from 28-30 Pf/ft in 50
ohm designs. For many demanding applications, far better
capacitance performance is desirable.
Expanded PTFE insulation, such as that which can be made in
accordance with U.S. Pat. No. 3,953,566 to Gore, has many desirable
properties over full density fluoropolymer insulations, including
lower dielectric constant, improved matrix tensile strength,
lighter weight, etc. Although expanded PTFE insulative material
provides improved dielectric performance, it generally has not been
used in many low signal applications because of triboelectric
capacitive effect between metallic elements (e.g., cable shield
and/or conductor) and the expanded PTFE insulation aggravated by
the presence of air entrapped within the expanded PTFE. This
condition can lead to "noisy" performance by the cable due to
static charges generated by the cable under flex.
Accordingly, it is a primary purpose of the present invention to
provide an improved low-noise electrical insulation which has low
dielectric constant.
It is another purpose of the present invention to provide an
improved electrical insulation which incorporates desirable
insulative properties of expanded PTFE while maintaining structural
integrity between layers of insulation.
It is still another purpose of the present invention to provide an
improved electrical insulation which has a reduced tendency to
generate triboelectric interference even without an added static
dissipating layer.
These and other purposes of the present invention will become
evident from review of the following specification.
SUMMARY OF THE INVENTION
The present invention is a low noise cable suitable for use in
sensitive electrical signal transmission. The cable of the present
invention employs an insulative layer of expanded
polytetrafluoroethylene (PTFE) which is bonded using an adhesive,
such as FEP and/or PFA, directly or indirectly to a surrounding
shield layer in order to maintain fixed relative position between
the insulative layer and the shield layer during use. The bonding
process produces a tightly coherent interface between the
insulative layer and the shield which is resistant to separation
and movement during use.
By eliminating separation of the PTFE insulative layer, "noise" in
the form of triboelectric currents is significantly reduced in the
cable of the present invention while retaining the beneficial
insulative and other qualities of expanded PTFE. The bonding
process of the present invention is so successful that for some
applications a low noise semi-conductive layer commonly used to
dissipate triboelectric currents in low noise cables may not be
necessary. In those instances where the semi-conductive layer is
provided, the bonding process may likewise be used with it to form
the insulative layer, the semi-conductive layer, and the shield
layer into a single coherent unit. A further improvement of the
present invention is to bond the conductor to the insulative layer,
providing an even more cohesive final cable.
The cable of the present invention provides a number of significant
benefits, including lower capacitance, smaller size, lighter
weight, improved flexibility, and reduced susceptibility to damage
during use. Further, the process of producing a low-noise cable is
likewise improved by the present invention, including reduced
manufacturing time and expense, ease in connector assembly and
reduced material costs.
DESCRIPTION OF THE DRAWINGS
The operation of the present invention should become apparent from
the following description when considered in conjunction with the
accompanying drawings, in which:
FIG. 1 is a three-quarter perspective view of one embodiment of a
cable of the present invention;
FIG. 2 is a cross-sectional view of the cable shown in FIG. 1;
FIG. 3 is a three-quarter perspective view of another embodiment of
a cable of the present invention;
FIG. 4 is a cross-sectional view of the cable shown in FIG. 3;
FIG. 5 is a three-quarter perspective view of still another
embodiment of a cable of the present invention; and
FIG. 6 is a cross-sectional view of the cable shown in FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises an improved low noise cable for the
transmission of electrical signals. While the cable of the present
invention is particularly suited for use in transmitting signals
between sensitive apparatus, the cables of the present invention
may be applied to virtually any application where electrical
signals must be conveyed accurately, such as wires for high gain
amplifiers, cables for oscilloscope probes, connectors for
piezoelectric components (e.g., microphones, accelerometers, eddy
current sensors), etc.
Shown in FIGS. 1 and 2 is a first embodiment of a cable 10 of the
present invention. Cable 10 comprises: a conductor 12, in this
instance a multiple strand conductor comprising seven strands of
silver plated copper wire; an insulative layer 14 of expanded
polytetrafluoroethylene (PTFE) surrounding the conductor, such as
expanded PTFE tape made in accordance with U.S. Pat. No. 3,953,566
issued Apr. 27, 1976, to Gore; a shield layer 16, such as a metal
braid shield material; and an outer insulative jacket 18, such as
expanded or fully density PTFE, fluorinated ethylenepropylene
(FEP), or perfluoroalkoxy polymer (PFA).
As has been noted, a problem with previous attempts to construct a
low noise cable in this manner has been that during use, and
particularly under conditions requiring substantial flexing or
vibration of the cabling during use, the expanded PTFE insulative
layer tends to separate from conductive shield layer 16 and/or the
conductor 12. Separation of this nature can then lead to the
generation of triboelectric currents when the insulation layer rubs
against the conductive elements during use. The "noise" generated
by this condition simply cannot be tolerated by highly sensitive
electronic equipment.
To correct this problem, the present invention employs an adhesive,
preferably a fluoropolymer adhesive, to bond at least the
insulative layer 14 and the shield layer 16 in fixed relative
position with each other. As is explained in detail below, this
bonding process may comprise direct adhesion between the insulative
layer and the shield layer or adhesion of both layers to an
intermediate structure. The purpose is to establish a coherent
structure which resists separation and relative movement between
the insulative layer and the shield layer during use.
Suitable fluoropolymer adhesives for use with the present invention
include fluorinated ethylenepropylene (FEP) and perfluoroalkoxy
polymer (PFA) (e.g., TEFLON FEP 100 or TE-9787 PFA dispersion, each
available from E. I. duPont de Nemours and Company, Wilmington,
Del.), as well as coated adhesives, such as polyesters,
polyurethanes, etc.
Preferably, the insulative layer and the shield layer are bonded
directly or indirectly together after the cable has been fully
assembled by applying heat or other activation energy to the cable.
In this manner, a firmly coherent structure can be provided which
is highly resistant to separation during use.
Most preferably, the cable is constructed in the following manner.
A non-expanded or slightly expanded (e.g., 2:1 expansion) substrate
of PTFE is laminated to a film layer of thermoplastic fluorinated
ethylenepropylene (FEP), such as FEP 100 available from E. I.
duPont and Company, to form a composite. Lamination should occur at
a temperature above the melt temperature of the thermoplastic. The
laminate is then further stretched at a temperature above the melt
of the thermoplastic (e.g., at a ratio of 2:1 to 100:1 or more),
drawing the composite down to a high strength material. One
suitable procedure for processing PTFE in this manner is disclosed
in U.S. Pat. No. 3,953,566 to Gore, incorporated by reference. This
produces a high strength composite with very thin layers of
expanded PTFE and FEP.
Additionally, or alternatively, the expanded PTFE may be treated
with a perfluoroalkoxy polymer (PFA) material, such as TE-9787 PFA
dispersion or a TEFLON PFA 340 material, each available from E. I.
duPont and Company. The PFA may be incorporated into the PTFE
structure in any suitable manner, such as by dry blending or
co-coagulation to make powders for dough or paste extrusion
processes.
Preferably, the PFA is incorporated through a procedure similar to
that disclosed in U.S. Pat. No. 4,985,296 to Mortimer, incorporated
by reference. Specifically, the PFA is combined in the following
manner. First, a PFA dispersion or powder is thoroughly mixed with
a PTFE dispersion or powder. If one or both of the components is a
dispersion, the mixed material is then dried to a powder form. The
resulting powder is then lubricated, such as with mineral spirits,
and then calendered, cross-calendered, extruded, or worked to a
desired thickness (e.g., 1 to 50 mils). The worked material can
then be stretched at an elevated temperature to produce an expanded
PTFE material. Finally, the lubricant is removed, such as through
evaporating at elevated temperature. Using this procedure, the PFA
material tends to become embedded within the nodal structure of the
expanded PTFE.
In addition to the above processes, a variety of other procedures
may be used to apply an adhesive to the PTFE material. Other
suitable procedures include: producing a PFA tape in a manner
similar to that described above concerning the production of an FEP
tape; applying an adhesive-filled laminate to the PTFE material to
act as an adhesive layer; filling the PTFE with FEP in the manner
similar to that described above as relating to PFA filling; or
combining one or more of the above procedures such as using both an
adhesive fill and an adhesive tape or laminate. Finally, although
PFA and FEP are the preferred adhesives for use with the present
invention, other adhesives may also be used, such as polyesters,
ethylene tetrafluoroethylene (ETFE) (e.g., TEFZEL polymer available
from E. I. duPont and Company).
While a high temperature adhesive such as PFA or FEP is preferred
for use in the present invention, it should be appreciated that any
thermoplastic or other adhesive system may be used with successful
results in low temperature or other less demanding
applications.
Once the expanded PTFE material is treated in the above described
manner, the cable is then fully assembled into its completed form.
Once assembled, bonding between the insulative layer 14 and shield
layer 16 (and the conductor 12, if desired) can then be readily
accomplished by simply applying heat to the finished cable to
activate the FEP and/or PFA materials. While the amount and time of
heat treatment is heavily dependent on the specific size and
characteristics of each cable, a heat treatment of 390.degree. to
430.degree. C. for 5 to 20 seconds is believed suitable for most
applications.
Insulative layers 14 which can be used in the present invention are
preferably a composite of adhesive and an expanded PTFE material,
such as those made in accordance with U.S. Pat. No. 3,953,566 to
Gore, incorporated by reference. In its easiest to use form, the
insulative layer comprises a tape cut from a sheet of expanded PTFE
material which is wrapped around the conductor (e.g., helically
wrapped, or longitudinally wrapped (i.e., in a cigarette fashion)).
Alternative insulative layers that may be used with the present
invention include foamed expanded FEP, extruded expanded PTFE,
expanded porous polyethylene, etc. Some of the advantages of using
one of these other insulative materials, such as FEP or
polyethylene, are: reduced material expense possibly more
uniformity; and possible elimination of need for a separate
adhesive material.
Shield layers 16 which can be used in the present invention include
any appropriate electrically conductive material that can be bonded
to the insulative layer. Among the suitable presently available
shielding materials are: braided metal shield; conductive polymer
shield; served wire shield; helically wrapped foil shield;
cigarette wrapped shield, and metallized film shield (e.g.,
aluminized polyester). Additionally, in some instances it may be
possible to provide a suitable bond by applying an adhesive layer
to the interior of the shield itself in order to establish the bond
with the insulative layer.
Cables made with the construction shown in FIGS. 1 and 2 have
demonstrated good low noise characteristics, even without use of a
low noise conductive layer commonly used to dissipate triboelectric
currents in low noise cables. Typical electrical properties for a
cable of this construction include: impedance of 55.+-.5 ohms; a
nominal capacitance of 23 pf/ft; velocity of propagation of 87% of
air; and a center conductor resistance of 0.097 ohms/ft.
Shown in FIGS. 3 and 4 is another construction of a cable 20 of the
present invention. In this instance, the cable comprises: a
conductor 22, such as multiple strand silver plated copper; an
insulative layer 24 of expanded PTFE; a low noise semi-conductive
layer 26, such as conductive particle filled PTFE (e.g., carbon
filled tape with or without an adhesive laminated on one side); a
shield layer 28; and an insulative jacket 30.
The construction of FIGS. 2 and 3 differs from that shown in FIGS.
1 and 2 primarily in the addition of the low noise semi-conductive
layer 26. While "noise" is vastly reduced through use of the cable
construction of the present invention, the inclusion of
semi-conductive layer 26 assures that a pathway is present for the
ready dissipation of any triboelectric currents that may be
generated during use of the cable.
The semi-conductive layer 26 may be constructed with a variety of
materials and incorporating a variety of different properties.
Examples of such layers include: dense carbon filled materials,
metal filled or metal plated materials, and undensified
(conformable) materials. Suitable materials for the semi-conductive
layer 26 include, without limitation, metal or carbon filled
expanded PTFE, and metal or carbon coated polyester or other
polymer film.
In the preferred construction of the cable shown in FIGS. 3 and 4,
prior to construction both the insulative layer 24 and the
semi-conductive layer 26 are laminated with a layer of FEP and/or
coated with a PFA material in the manner previously described. Once
the cable 20 is assembled into its final form, heat or other
activation energy can be applied to bond the conductor 22, the
insulative layer 24, the semi-conductive layer 26, and the shield
layer 28 into a coherent unit.
Typical electrical properties for a cable of this construction
include: impedance of 50.+-.5 ohms; capacitance of 29 pf/ft;
nominal velocity of propagation of 75% of the speed of light; and
center conductor resistance of 0.097 ohms/ft.
Shown in FIGS. 5 and 6 is still another embodiment of a cable 32 of
the present invention. This construction comprises: a single strand
conductor 34; a PFA filled expanded PTFE insulative layer 36; a low
noise conductive layer 38; a shield layer 40; and an insulative
jacket 42. The application of heat to the PFA filled expanded PTFE
insulative layer 36 serves to bond the conductor 34, the insulative
layer 36, and the semi-conductive layer 38 into a coherent
unit.
Typical electrical properties for a cable of this construction
include: impedance of 50.+-.5 ohms; nominal capacitance of 25
pf/ft; and nominal velocity of propagation of 75% of the speed of
light.
Without intending the limit the scope of the present invention, the
following examples demonstrate how the present invention may be
made and used:
EXAMPLE 1
An expanded PTFE tape was prepared with a filling of PFA adhesive
in the following manner:
A 0.25 lb quantity of PFA350 powder (80 micron), acquired from E.
I. duPont and Company, was added to 625 cc of mineral spirits and
blended for about 1 minute. This mixture was then added to 5 lbs of
TE-3525 PTFE fine power, acquired from E. I. duPont and Company,
and blended for approximately 8 minutes. A preform was then made
from this blended material, applying about 25 in Hg vacuum and 430
psi pressure. The preform was then ram extruded at ambient
conditions through a flat die at 2326 psi hydraulic pressure to
form a tape.
Once the tape was formed, it was calendered in two passes through
heated rolls to 3.8 mils thickness. The mineral spirits were then
dried across heated rolls (maximum temperature about 300.degree.
C.) and the tape was stretched simultaneously at a maximum
temperature of about 225.degree. C. at a ratio of 4:1 and an output
speed of 130 ft/min. Finally, the tape was sintered at 369.degree.
C. across heated rollers at 130 ft/min and at a ratio of 1:1. The
resulting tapes had a bulk density of 0.675 g/cc and had an average
thickness of 2.9 mils.
The PFA filled tape was then helically wrapped as an insulative
layer over a silver plated copper wire (AWG 30 (7/38)) to a
diameter of 0.032" (0.081 cm). Using a conventional braiding
machine, a braided shield, comprising a AWG 38.sup.(1) SPC wire
with 20 picks per inch and 3 ends, was wrapped over the taped
wrapped wire. A closing die was used to apply compression. The
braided cable had a 0.047" (0.119 cm) outside diameter (OD). The
shielded cable was then exposed to heat in a convection oven of
410.degree. C. for about 10 seconds to activate the adhesive.
Following heat treatment, an extruded PFA jacket was then added
with a wall thickness of 0.010" (0.025 cm), producing a final cable
OD of 0.067" (0.17 cm).
The final cable had the following properties:
Impedance of 55.+-.5 ohms
Velocity of Propagation of 87%
Capacitance of 23 pico farads/ft
Noise test (voltage) of 0 millivolts
Noise test (current) of 1.94 picoamps
The noise tests for voltage and current were carried out in
accordance with the following procedure. A "bowstring" type
excitation apparatus was built to support a length of cable under
tension between two ends. A connector was mounted on one end of a
sample cable at least five feet long and the other end of the cable
was left exposed. The connector was then attached to a Keithly
Model 617 Programmable Electrometer. The electrometer was in turn
connected to a Tektronic TDS 540 Digital Storage Oscilloscope.
A set amount of weight was then applied to the exposed end of the
cable to place tension upon it. Care was taken to assure that the
cable was not in electrical contact with any conductive surfaces
and that the conductor and the shield of the cable were not in
touch with each other. A cable support post was then placed mid-way
between the two ends of the cable to stretch the cable out of the
plane between its two ends. The support post was adapted to be
removed to release the cable and allow it to vibrate freely between
its two ends.
At this stage, the electrometer was calibrated to zero and the
oscilloscope was set to record the wave form generated by the
vibrating cable. The cable support post was then released, allowing
the cable to vibrate freely, and electrical readings were
taken.
The response recorded on the oscilloscope was directly related to
the current measured by the electrometer as a function of time. If
the electrometer is in the 2 picoamp range, the voltage waveform
displayed on the oscilloscope represents 1 picoamp current for one
volt on the oscilloscope (i.e., there is a 2:1 ratio between
electrometer range in picoamps and oscilloscope current range in
picoamps for each one volt on the oscilloscope). If the voltage
displayed on the oscilloscope is greater than 2 volts, the
electrometer was switched to the next higher range and the test was
repeated.
EXAMPLE 2
An insulative layer of PFA filled PTFE tape made in accordance with
Example 1 was helically wrapped over an AWG 30(7/38) SPC wire to an
OD of 0.034" (0.086 cm). A semi-conductive layer comprising an FEP
laminated carbon filled PTFE tape was then helically wrapped over
the PFA filled tape to an OD of 0.040" (0.102 cm).
The FEP laminated carbon filled PTFE tape was made in the following
manner. A slurry of 1574 g of Ketjenblack 300-J carbon black
obtained from AKZO Chemicals was slurried with 55.0 I of deionized
water in a 30 gallon baffled stainless steel vessel. While the
slurry was agitating at 120 rpm, 4633 g of PTFE in the form of a
15.2% dispersion was rapidly poured into the mixing vessel. The
PTFE dispersion was an aqueous dispersion obtained from ICI
Americas Co. The mixture was self-coagulating and within 1.5
minutes co-coagulation was complete. After 10 minutes, the coagulum
had settled to the bottom of the mixing vessel and the water was
clear.
The coagulum was dried at 165.degree. C. in a convection oven. The
material dried in small, cracked cakes approximately 2 cm thick and
was chilled to below 0.degree. C. The chilled cake was hand ground
using a tight, circular motion and minimal downward force through a
0.635 cm mesh screen. The resulting powder was lubricated using
1.24 cc of mineral spirits per gram of coagulum. The lubricated
mixture was chilled, passed through a 0.065 cm mesh screen, tumbled
for 10 minutes, allowed to sit at 18.degree. C. for 48 hours, then
re-tumbled for 10 minutes.
A pellet was then formed in a cylinder by pulling a vacuum and
pressing at 800 psi. The pellet was heated in a sealed tube and
extruded into a tape form.
The tape was then calendered through heated rolls to approximately
10.5 mils. The lubricant was evaporated by running the tape across
heated rolls (at 270.degree. C.).
The tape was then expanded at 105 ft/min output speed across heated
rollers (at 270.degree. C.) at a ratio of 3:1. The material was
laminated to 0.5 mil FEP-100 film across a heated surface while
stretching twice at a ratio of 1.3:1 and 1.2:1 at 335.degree. C.
and an output speed of 30 ft/min. The bulk density of the tape was
0.187 g/cc and was approximately 6 mils thick.
A wire braid shield layer was then installed in the manner
described in Example 1 (AWG 38.sup.(1) SPC braid with 20 picks per
inch and 3 ends) and a closing die was employed to apply
compression to an OD of 0.054" (0.137 cm).
Heat treatment was then performed in accordance with Example 1 to
activate the adhesive material and bond the insulative layer,
semi-conductive layer and shield layer together. Following heat
treatment, an extruded jacket of PFA was then installed to produce
a finished diameter of 0.070" (0.178 cm).
The final cable had the following properties:
Impedance of 55.+-.5 ohms
Velocity of Propagation of 75%
Capacitance of 25 pico farads/ft
Noise test (voltage) of 6.24 millivolts
Noise test (current) of 2.48 picoamps
EXAMPLE 3
An expanded PTFE tape was prepared with a coating of FEP adhesive.
The process set forth in Example 1, above, was followed using 63
lbs of TE-3525 resin blended with 7875 cc of mineral spirits. In
the expansion step, the tape was expanded at a ratio of 3.8:1. The
expanded tape was laminated to 0.5 mil FEP-100 film across heated
surfaces at a temperature of 315.degree. C., a ratio of 1.25:1, and
a 46 ft/min output speed. The material was run through heated
plates at the same conditions two more times to yield a final bulk
density of 0.51 g/cc and 3.1 mils thickness.
The FEP laminated expanded PTFE tape then was helically wrapped as
an insulative layer over a silver plated copper wire (AWG 30(7/38)
to a diameter of 0.033" (0.084 cm). Using a conventional braiding
machine, a braided shield, comprising a AWG 38.sup.(1) SPC wire
with 20 picks per inch and 3 ends, was wrapped over the taped
wrapped wire. A closing die was used to apply compression. The
braided cable had a 0.056" (0.142 cm) OD. The shielded cable was
then exposed to heat in a convection oven of 410.degree. C. for
about 10 seconds to activate the adhesive.
Following heat treatment, an extruded PFA jacket was then added
with a wall thickness of 0.010" (0.025 cm), producing a final cable
OD of 0.072" (0.18 cm).
The final cable had the following properties:
Impedance of 50.+-.5 ohms
Velocity of Propagation of 75%
Capacitance of 29 pico farads/ft
Noise test (voltage) of 37.5 millivolts
Noise test (current) of 2.16 picoamps
EXAMPLE 4
An insulative layer of FEP laminated expanded PTFE tape made in
accordance with Example 3 was helically wrapped over an AWG
36(7/44) CS-95 wire to an OD of 0.016" (0.041 cm). A wire braid
shield layer was then installed in the manner described in Example
3 only applying a AWG 44.sup.(1) SPC wire at 30 picks per inch and
3 ends. A closing die was employed to apply compression to an OD of
0.022".
An expanded PTFE tape was then helically wrapped over the braided
cable and heated with a contact heater at 410.degree. C. for about
5 seconds was applied to sinter the ePTFE and activate the
adhesive. The final diameter of the cable was 0.030" maximum.
The final cable had the following properties:
Impedance of 50.+-.5 ohms
Capacitance of 25 pico farads/ft
Noise test (voltage) of 0.44 mVolts
Noise test (current) of 1.51 picoamps
EXAMPLE 5
It is contemplated that the present invention may likewise be
practiced with a variety of other materials serving as the
insulative layer. One form envisioned for the present invention
comprises forming an insulative layer from a foamed polymer
material such as FEP, PFA, or ethylene-tetrafluoroethylene (ETFE).
This may be effective in dropping the dielectric constant of the
insulation from 2.2 toward 1. The ultimate insulative properties
achieved are dependent upon a number of factors, including the
material used and the final air content (density) of the material.
The following are examples of how the present invention may be
practiced using such materials.
Continuous foaming of FEP, PFA, or ETFE resin can be achieved by
using a blowing agent (e.g., FREON 22 fluoromethane gas available
from E. I. duPont de Nemours and Company) and an extruder. Suitable
polymers for use in this process include FEP 100, PFA 340, and
CX5010 polymers, all available from E. I. duPont de Nemours and
Company. Foaming of the insulation material should be carried out
in accordance with the polymer manufacturer's instructions. The
following is an outline of suitable procedures for the above listed
preferred polymers acquired from E. I. duPont de Nemours and
Company.
The blowing agent is dissolved in the resin to equilibrium
concentrations, such as by injection in a screw extruder. By
adjusting the pressure in the extruder, the amount of blowing agent
dissolved in the melt can be controlled. The greater the amount of
blowing agent dissolved in the melt, the greater the final void
volume of the foam.
For use in the present invention, a single screw extruder, such as
that available from Entwistle Company, Hudson, Mass., provided with
a medium size screw (e.g., 1.25), should be suitable. Preferably a
"super shear" extrusion process should be used to reduce the
temperature of die to about 45.degree. C. below the melt
temperature of the resin. Ideally, a five zone extruder should be
employed to provide uniform blowing agent dispersion. Other
preferred operating parameters include: providing a die with a
relatively long land; using either fixed centered or
adjustable-centered crossheads; employing shallow entry cone
angles; providing careful temperature control, and employing
smooth, streamlined tooling (both tip and die); and using high
nickel alloy crosshead components. The tip and die size should be
appropriately selected for wire and wall thickness (e.g., a AWG
30(7/38) 0.012" conductor with a wall thickness of about 0.0125").
A vacuum should be applied from the rear of the crosshead to pull
the insulation tightly onto the conductor.
Foam formation begins as the molt resin passes out of the extrusion
die. The blowing agent dissolved in the polymer resin comes out of
the resin as a result of sudden pressure drop as the extrudate
exits the extrusion die. Foam growth ceases upon cooling, such as
when the extrudate enters a water cooling trough.
To produce uniform, small diameter cell structure, a nucleating
agent may be employed, such as boron nitride. A 0.5% by weight
loading of boron nitride should provide adequate foam cell
nucleation. This level of nucleating agent loading can be achieved
by blending a cube concentrate resin FEP or PFA containing 5% boron
nitride with virgin, unfilled resin. A cube blend of 1 part
concentrate to 9 parts untilled resin will approximate the 0.5%
loading. Concentrate resins are commercially available in this
form.
The amount of foaming which occurs exiting the extruder is a
function of the temperature of the crosshead and should be
carefully controlled. Additionally, capacitance and the diameter of
the insulation should likewise be continuously monitored as it
exits the extruder to assure uniformity.
It is also possible to purchase conductors with a foamed polymer
adhesive pre-applied to them, such as foamed polyethylene or foamed
polypropylene. Such material is commercially available under the
trademark BRAND REX from Brintec Systems Corporation, although use
of such conductors in the present invention may require stripping
of the jacket and shielding from pre-formed cables.
Once a foamed insulation is applied to a conductor in the manner
described above, the wire may then be incorporated into a cable of
the present invention. For example, using a AWG 30(7/38) conductor
and a fluoropolymer such as FEP or PFA foam with a diameter of
0.036" over the conductor, a braid is applied. One suitable form
comprises using AWG 38.sup.(1) SPC using a standard 16 carrier
Wardwell braiding machines using 3 ends and 20 picks per inch. A
closing die 0.051" may be used to apply the braid with
compression.
Since the insulation itself is an FEP, PFA, or ETFE polymer
material that can also serve as an adhesive, at this stage the
entire assembly may be heat treated (e.g., using a convection oven
or contact heater) at a temperature of about 390.degree. to
410.degree. C. for 5-20 seconds to bind the insulative layer to the
braided shield layer. Additionally, a separate fluoropolymer
adhesive may also be applied within the cable to provide even
stronger adhesion.
A jacket may then be applied by any suitable method. For instance,
a jacket may be applied by wrapping several layers of PTFE jacket
material (e.g., three layers of 0.003" thick PTFE tape) on to the
braided cable and then sinter it at 390.degree. C. for about 10
seconds in a convection oven. Alternatively, a wall of PFA (e.g.,
0.007") may be extruded around the braid using standard extrusion
technology. The finished diameter is approximately 0.065".
Producing a cable in this manner should produce a cable with
approximately the following properties:
Impedance approximately 55.+-.5 ohms
Velocity of Propagation approximately 82%
Capacitance approximately 24 pf/ft
Low Noise Performance
EXAMPLE 6
A fluoropolymer foam insulation (e.g., FEP, PFA, or ETFE) may be
applied to a wire in the manner described in Example 5, for
instance, using an AWG 30(7/38) wire with an insulated diameter of
0.036". An FEP laminated carbon filled PTFE semi-conductive layer
may then be applied to the insulated conductor by wrapping in a
helical manner, to bring the diameter to about 0.043". On top of
the semi-conductive layer, an AWG 38(.sup.(1) braid may be applied
using 3 ends and 20 picks per inch on a 16 carrier Wardwell
braider. A closing die of 0.058" should be used to apply
compression. The braided diameter will be 0.059". Once formed in
this manner, the insulative layer may be bonded to the shield layer
by applying 390.degree. to 410.degree. C. heat for 5 to 20 seconds.
A jacket layer may then be applied, such as through helically
wrapping the PTFE to the braided cable and applying 390.degree. C.
heat for 10 seconds in a convection oven or by applying a 0.007"
wall of PFA using standard extrusion technology. The finished
diameter should be approximately 0.073".
Producing a cable in this manner should produce a cable with
approximately the following properties:
Impedance approximately 50.+-.5 ohms
Velocity of Propagation approximately 73%
Capacitance approximately 27 pf/ft
Low Noise Performance
EXAMPLE 7
Another insulation that may be used in the present invention
comprises a polyolefin foam insulation, such as a polyethylene.
This insulation can be manufactured using conventional extrusion
equipment, such as a 1.5" Entwistle plastic extruder. Pressure
extrusion tooling works best and should be selected based on
substrate diameter and desired wall thickness. A resin containing a
blowing agent is used for this type of extruding. Suitable material
can be purchased from a number of sources, such as Quantum Chemical
Corporation, Cincinnati, Ohio, under the trademark PETROTHENE. This
resin incorporates a compatible blowing agent. Barrel pressures are
kept at about 750 to 1200 lb/in.sup.2. A typical temperature
profile is:
______________________________________ Extruder Zone Feed 2 3 4-X
Adapter ______________________________________ Temp. 320.degree.
330.degree. 350.degree. 390.degree. 400.degree. (.degree.F.)
______________________________________
The blowing agent must be activated with the higher temperature as
it approaches the adapter (cross-head) exit. This temperature
setting must be precisely controlled to maintain consistency of
foam density. Temperature can also can be used as a controlling
variable for foam density.
This polymer foam insulation may be applied to a wire in the manner
described in Example 5, for instance, using an AWG 30(7/38) wire
and applying the low dielectric polyethylene insulating foam to a
diameter of approximately 0.037". This foam insulation will have a
dielectric constant of approximately 1.7. To this insulated
conductor an AWG 38.sup.(1) braid may be applied in the manner
previously described. A 0.054" closing die may be used in order to
apply the braid with compression. Adhesion of the insulative layer
to the shield layer can be achieved by applying 300.degree. C. heat
for about 5-20 seconds in a convection oven. The diameter of the
braided cable should be approximately 0.055". A jacket may then be
applied in the manner previously described to produce a finished
cable with a diameter of approximately 0.069".
Producing a cable in this manner should produce a cable with
approximately the following properties:
Impedance approximately 55.+-.5 ohms
Velocity of Propagation approximately 81%
Capacitance approximately 26 pf/ft
Low Noise Performance
EXAMPLE 8
A polyethylene foam insulation may be applied to a wire in the
manner described in Example 7, for instance, using an AWG 30(7/38)
wire and applying a low dielectric polyethylene foam insulation to
a diameter of approximately 0.037". This foam insulation will have
a dielectric constant of approximately 1.7. A semi-conductive layer
may then be applied, such as by helically wrapping a FEP laminated
carbon filled PTFE to a diameter of about 0.044". An AWG 38.sup.(1)
braid may then be applied in the manner previously described. A
closing die of 0.056" may be used to apply the braid under
compression. A rapid heat treatment at 390.degree.-410.degree. C.
may then be performed to bond the insulative layer to the shield.
Again, a jacket may be applied, such as by helically wrapping and
sintering a PTFE tape or extruding a 0.007" wall of PFA. The
finished cable diameter should be approximately 0.075".
Producing a cable in this manner should produce a cable with
approximately the following properties:
Impedance approximately 50.+-.5 ohms
Velocity of Propagation approximately 75%
Capacitance approximately 30 pf/ft
Low Noise Performance
While particular embodiments of the present invention have been
illustrated and described herein, the present invention should not
be limited to such illustrations and descriptions. It should be
apparent that changes and modifications may be incorporated and
embodied as part of the present invention within the scope of the
following claims.
* * * * *